ci/cd: build vcpkg ports release-only via an overlay triplet

vcpkg builds every port twice (release + debug) in manifest mode and the
VCPKG_BUILD_TYPE environment variable is ignored there; pin the build
type in overlay triplets instead (tooling/vcpkg-triplets/release, one per
supported triplet). The release configuration is what both workflows
consume: FFMPEG_DIR drives the release layout (lib/pkgconfig) and the C
ABI is identical for the debug binary CI tests. Verified locally in
debian:12: a release-only overlay builds zlib/libpng/openssl/pkgconf/
libsndfile with zero debug trees.

Debug-only is not an option: ports like zlib patch files in the release
layout and fail when only the debug configuration is built.

.github/workflows/cd.yml

@@ -274,7 +274,7 @@ jobs:
           # whole install — vcpkg refuses to retry that class of curl
           # error — so retry here.
           for attempt in 1 2 3; do
-            vcpkg install --triplet ${{ matrix.triplet }} && exit 0
+            vcpkg install --triplet ${{ matrix.triplet }} --overlay-triplets tooling/vcpkg-triplets/release && exit 0
             echo "vcpkg install failed (attempt $attempt); retrying"
             sleep 15
           done
@@ -285,7 +285,7 @@ jobs:
         shell: pwsh
         run: |
           for ($i = 1; $i -le 3; $i++) {
-            vcpkg install --triplet ${{ matrix.triplet }}
+            vcpkg install --triplet ${{ matrix.triplet }} --overlay-triplets tooling/vcpkg-triplets/release
             if ($LASTEXITCODE -eq 0) { exit 0 }
             Write-Host "vcpkg install failed (attempt $i); retrying"
             Start-Sleep -Seconds 15